Dynamic isolating mount for processor packages

ABSTRACT

The present invention relates to a method and apparatus that prevents/minimizes cracking in the ceramic body of processors. The ability to prevent/minimize cracking can ensure successful operation and substantially increase processor lifetime. The present invention discloses a device for maintaining a microprocessor in a desired relationship with a printed wiring board while limiting the transmission of shock and vibrational motion to and from the processor includes a printed wiring board, a processor, and a dynamic isolating mount compressed between the printed wiring board and the processor, wherein the processor maintains the dynamic isolating mount in a compressed state such that the dynamic isolating mount bears on the printed wiring board.

CROSS-REFERENCE TO RELATED APPLICATIONS

The subject matter of the present application is related to the subjectmatter of co-pending application entitled “Tunable Vibration Damper forProcessor Packages,” incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to protecting microprocessorsfrom overstress caused by sudden forces or impacts. More particularly,the present invention relates to a dynamic isolating mount for amicroprocessor package.

2. Background of the Invention

For many decades, circuit boards, such as those used in computers, havebeen manufactured by attaching electrical components to the board. Insome cases, the components are soldered directly to the board. Althoughgenerally satisfactory, soldering a component directly to the boardmakes it difficult and costly to change that component should it bedesired or necessary to replace one component with another. Amicroprocessor, for example, may have hundreds of connections that,should the processor fail, must be desoldered. A new processor, with itshundred of connections must then be attached to the board. Further, thisprocess must occur without damaging the other components mounted on thecircuit board. Even if the processor has not failed, it still might bedesired to replace it, for example, a new and improved version of theprocessor is made available.

For these and other reasons, “interposer” sockets have became available.Although defined in various ways, an interposer socket is a socket towhich a chip (i.e., a microprocessor) is mated. The socket is then matedto the circuit board or to a socket soldered to the circuit board.Advantageously, an interposer socket does not require solder either tobe mated to the board (or other socket) or to the electrical componentmounted on it. Instead, a lever or other mechanism is engaged to holdthe interposer socket to the circuit board.

As technology has progressed, some chips (i.e., microprocessors) havebecome more powerful and accordingly consume more electrical power. Thisincrease in power usage causes the chips to become hotter and largerheat sinks are required to dissipate the increased thermal load.Mounting a large chip with a heat sink in an interposer socket may beproblematic in the face of shock/vibration loads.

For example, motion caused by a fan, opening and closing cabinet doorsin a rack of computers, seismic activity, and vibration induced byadjacent equipment may cause the ceramic body of a chip to crack andultimately fail. Obviously, this failure may cause the electricalcomponent contained in the interposer to cease functioning as intended.

For successful operation and prevention of premature chip failure, thesource of the vibration should be eliminated. If this is impossible ordifficult, then a vibration isolation device should be used at or nearthe socket to minimize the potential for the chip to fail.

BRIEF SUMMARY OF THE INVENTION

The present invention relates to a method and apparatus thatprevents/minimizes cracking in the ceramic body of chips. The ability toprevent/minimize cracking can ensure successful operation andsubstantially increase processor lifetime.

In accordance with a preferred embodiment of the present invention, adevice for maintaining a microprocessor in a desired relationship with aprinted wiring board while limiting the transmission of shock andvibrational motion to and from the processor includes a printed wiringboard, a processor, and a dynamic isolating mount compressed between theprinted wiring board and the processor, wherein the processor maintainsthe dynamic isolating mount in a compressed state such that the dynamicisolating mount bears on the printed wiring board.

In accordance with another preferred embodiment of the presentinvention, a method for limiting shock/vibrational motion of amicroprocessor includes placing a dynamic isolating mount between aprocessor and printed wiring board.

In accordance with yet another preferred embodiment of the presentinvention, a method for preventing cracking of the ceramic body of amicroprocessor includes placing a dynamic isolating mount on a printedwiring board where the printed wiring board contacts the processor.

These and other aspects of the present invention will become apparentupon studying the following detailed description, figures and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of the preferred embodiments of theinvention, reference will now be made to the accompanying drawings inwhich:

FIG. 1 is a detailed schematic diagram of a system in accordance with apreferred embodiment of the present invention;

FIG. 2 is a schematic diagram of a spring-dashpot model; and

FIG. 3 is a simplified schematic diagram of a system in accordance witha preferred embodiment of the present invention.

NOTATION AND NOMENCLATURE

Certain terms are used throughout the following description and claimsto refer to particular system components. As one skilled in the art willappreciate, computer companies may refer to a given component bydifferent names. This document does not intend to distinguish betweencomponents that differ in name but not function. In the followingdiscussion and in the claims, the terms “including” and “comprising” areused in an open-ended fashion, and thus should be interpreted to mean“including, but not limited to . . . ” Also, the term “couple” or“couples” is intended to mean either an indirect or direct electricalconnection. Thus, if a first device “couples” to a second device, thatconnection may be through a direct electrical connection, or through anindirect electrical connection via other devices and connections. To theextent that any term is not specially defined in this specification, theintent is that the term is to be given its plain and ordinary meaning.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Currently, there is desire to mount CPUs in area array interposersockets. The manner of mounting a CPU in an interposer socket requiressupporting the ceramic body at discrete locations around the peripheryof the device. Because the ceramic body of high performance processorsis brittle, these support points can then become origins for cracks whenthe device is subjected to assembly and impact type forces. The presentinvention provides a positive means for controlling cracking in theceramic body of the processor by providing assembly support andshock/vibration isolation through an energy dissipation device, ordynamic isolating mount.

The preferred embodiment of the invention is described below in thecontext of a processor chip and heat sink combination mounted on acircuit board with an interposer socket. It should be noted, however,that the chip need not be a processor nor is the heat sink required.Broadly, the invention is useful to reduce vibration for any type ofcomponent mounted to a circuit board.

Referring initially to FIG. 1, system 100 includes a backing plate 10with a plurality of recesses 20, a PWB 30, interposer socket 40, alandgrid array (LGA) or processor 42, interposer columns 45, a heat sink55, and at least one dynamic isolating mount 65. When combined,interposer columns 45 and processor 42 are referred to as CPU assembly50. As is known in the art, heat sink 55 is held in place by an assembly60 in such a manner as to achieve sufficient thermal contact therewith.If desired, a material such as thermal grease (not shown) can be used atthe interface to enhance the flow of heat from CPU assembly into heatsink 55. Assembly 60 preferably comprises a standoff 90, a helical coilspring 120, and a capscrew 110. Each standoff 90 is mounted on orpressed into backing plate 10 and passes through a corresponding hole 57in the base of heat sink 55. Standoff 90 each preferably comprise agenerally cylindrical member having a threaded internal bore 92. Eachcapscrew 110 includes a head 112 and a male threaded body 114 sized tothreadingly engage bore 92.

A plurality of interposer columns 45 connects PWB 30 to CPU assembly 50.While this configuration has many benefits associated with it,interposer columns 45 transfer vibrational energy from PWB 30 to CPUassembly 50.

According to a preferred embodiment, dynamic isolating mount 65 isplaced between PWB 30 and CPU assembly 50. Prior to engagement of CPUassembly 50 with PWB 30, dynamic isolating mount 65 is placed on PWB 30where PWB 30 contacts CPU assembly 50. Dynamic isolating mount 65 maycomprise a continuous piece or small, fragmented pieces. Dynamicisolating mount 65 is preferably somewhat taller than the space betweenPWB 30 and CPU assembly 50 when it is in its natural or non-compressedstate. Thus, when it is desirable to assemble system 100, CPU assembly50 is placed on top of dynamic isolating mount and secured in place bymethods known by one of ordinary skill in the art. In this manner, adynamic isolating force is applied to CPU assembly 50 to maintain it incontact with the PWB while simultaneously damping vibrations or shocksthat would otherwise be transmitted from the PWB to the CPU.

In order to describe the operation of the present invention, referencewill briefly be made to FIG. 2, a schematic of a simple spring-dashpotsystem. FIG. 2 includes a Kelvin element comprising a linear spring inparallel with a viscous damper. Kelvin model 200 includes a springcomponent 210, a dashpot component 220, and a fixed origin 230. In aKelvin model, spring component 210 functions according to Hookeanelastic behavior. For example, when a force is applied to spring 210 itdeforms by an amount that is directly proportional to the applied force.The classical solid behavior is given by Equation 1.

F=kx  (1)

where F is force (stress), x is the extension distance (strain), and kis the proportionality constant. This constant is also called a modulus.The deformation is reversible when the stress is removed. However, ifstress is continuously applied, a Hookean solid does not deform anyfurther; it shows no time-dependant deformation.

Dashpot, or damper 220, functions according to Newtonian viscousbehavior. For example, the applied force (stress) is proportional not tothe distance (strain), but rather to the rate of strain. This classicalviscous behavior is given by Equation 2.

F=kdx  (2)

where F is force, dx is the rate of extension (strain), and k is theproportionality constant. In shear this equation is written:

τ=ηγ  (3)

and the proportionality constant η is viscosity. The damping materialcontinues to deform as long as force is applied. The deformation is notreversible; when the force is removed, the damping material ceases todeform.

It should be understood that the only material that exhibits trueNewtonian viscous behavior is a viscous liquid. In reality, a “viscous”solid displays viscous and elastic behavior. However, for explanatorypurposes only, in the current invention, the interposer columns 40 areassumed to display purely elastic behavior and the dynamic isolatingmount is assumed to display purely viscous behavior.

When these two components are combined, the viscoelastic behavior of thesystem can be modeled using the elastic and viscous elements inparallel; the strain of the two elements in parallel is the same and thetotal stress is the sum of the stress in the two elements. As the loadis applied, the viscous element resists deformation but slowly deforms,transferring the applied stress to the elastic element. Thus, thedeformation of this two-element model is limited by the extensibility ofthe elastic element. When load is removed, the “transient creep” strainis recovered.

More specifically, this model exhibits a “delayed elastic” orviscoelastic response to applied loads. After sudden imposition of ashear stress, spring 210 will eventually reach the expected strain, butis retarded in doing so by dashpot 220. Dashpot or dynamic isolatingmount 65 of the present invention accordingly prevents column 45 fromreaching its expected strain, thus limiting vibrational motion.

In order to ensure that vibrational motion is minimized, dynamicisolating mount 65 should possess the following properties. It should beresistant to temperatures is below 130° C., possess a loss factor of atleast 0.010, and be easily manufactured by companies such as Sorbothane.Examples of such materials include, but are not limited to, rubbers,silicones, and neoprenes.

The simple Kelvin model described above describes a simplespring-dashpot system which is useful to understanding the followingmodel which more accurately models the behavior of dynamic isolatingmount 65.

Referring now to FIG. 3, spring-mass-dashpot system 300 preferablyincludes a heat sink assembly-CPU package 57, interposer columns 45, aPWB 30, and a dynamic isolator 65. Interposer columns 45 possess a totalspring constant K (lb/in), heat sink assembly-CPU package 57 possessesmass W/g (lb-sec²/in), and dynamic isolator 65 possesses a damping valueC (lb-sec/in). The magnification factor of a single degree of freedomspring-mass-damper system can be determined according to Equation 4:

X/X ₀=1/[{1−(ω/ω_(n))²}²+{2ξ(ω/ω_(n))}²]^(1/2)  (4)

where:

X is the amplitude of vibration (in),

X₀ is static deflection, or F₀/K (in),

ω is frequency of excitation (rad/sec),

ω_(n) is natural frequency, or [Kg/W]^(1/2) (rad/sec),

ξ is a damping factor, =C/C₀,

C₀ is critical damping, =2Wω_(n)/g (lb-sec/in),

k is the spring constant of one clamping spring (lb/in),

K is the total clamping spring constant, or nK (lb/in),

n is the number of clamping springs, in this case 4,

W is the weight of heat sink (lb),

g is a gravitational constant, or 386 in/sec², and

F₀ is the total static clamping force applied (lb).

For a resonant, critically damped system ξ=1, and Equation 4 becomesX/X₀=0.5. Thus, for a critically damped isolator, the dynamic amplitudewill equal half of the static compression of interposer columns. Sincethe columns will equal eventually compress approximately 0.010″, theabove analysis suggests that a critically damped dynamic isolator willprevent dynamic motion greater than 0.005″. Thus, using a criticallydamped isolator, 0.005″ of compression is attainable.

The critical damping value of the dynamic isolator can be determined byEquation 5:

C ₀=2Wω _(n)=2[KW/g] ^(1/2)  (5)

Thus, Equation 5 defines the amount of damping necessary in the dynamicisolator to provide a critically damped system.

Critical damping refers to zero amplitude for a damped oscillator; thebody returns back to its equilibrium position at an optimum rate.Critical damping is desirable because vibrational oscillations cease,preventing intermittent motion. By tuning the dynamic isolating mount 65to equal approximately twice the product of the mass weight and naturalfrequency, critical damping is obtained.

In order to tune a dynamic isolating mount, a critical damping value ismathematically projected, similar to that shown in Equation 5. Amaterial possessing a damping value equal to a fraction of the projectedcritical damping value is then employed as the damper. For example,according to Equation 5, if the weight of the heat sink is 0.10 lb andthe natural frequency of the system is 500 rad/sec, then the criticaldamping value, C₀ is 100 lb-rad/sec, because C₀=2Wω_(n). If two dynamicisolating mounts are used, each mount should possess a damping factor Cof approximately 50 lb-rad/sec.

The dynamic isolating mount may be produced in the form of apicture-frame, square tabs, or any form capable of damping theinterposer columns, including incorporating the dynamic isolating mountinto the interposer socket. Additionally, the dynamic isolating mountmay be part of a Kelvin system as described above (e.g., spring anddashpot in parallel) or part of a Maxwell system (e.g., spring anddashpot in series) and the spring(s) and damper(s) need not necessarilybe positioned adjacent to each other.

It should be understood that the damping assemblies and systemsdescribed herein may be used in a computer system including a chassis, asystem board, and an input device. In a preferred embodiment, the inputdevice is either a mouse or a keyboard.

The above discussion is meant to be illustrative of the principles andvarious embodiments of the present invention. Numerous variations andmodifications will become apparent to those skilled in the art once theabove disclosure is fully appreciated. It is intended that the followingclaims be interpreted to embrace all such variations and modifications.

What is claimed is:
 1. A device for maintaining a microprocessor in adesired relationship with a printed wiring board while limiting thetransmission of shock and vibrational motion to and from the processor,comprising: a printed wiring board; a processor; and a dynamic isolatingmount compressed between said printed wiring board and said processor;said processor maintaining said dynamic isolating mount in a compressedstate such that said dynamic isolation mount bears on said printedwiring board and such that the dynamic isolating mount is criticallydamped.
 2. The device of claim 1 further comprising interposer columns.3. The device of claim 1 wherein said dynamic isolating mount comprisesa continuous piece that contacts the periphery of the processor.
 4. Thedevice of claim 1 wherein said dynamic isolating mount comprises apredetermined number of pieces that contact the periphery of theprocessor.
 5. The device of claim 1 wherein said dynamic isolating mountcomprises material resistant to temperatures below 130° C.
 6. The deviceof claim 1 wherein the dynamic isolating mount possesses a loss factorat least 0.010.
 7. The device of claim 1 wherein the dynamic isolatingmount is manufactured by Sorbothane.